Electric powertrains are devices to transfer energy from an electric power source like a battery to a torque exerted by the tires of a vehicle and also if possible to feed some of the kinetic energy back to the battery while braking. The efficiency of these systems have however been so low that an increase thereof could make a substantial improvement of the range of electric vehicles. The present art can be illustrated by the ETX-II powertrain as presented on the 10th Electric Vehicle Symposium in Hongkong during authumn 1990. The ETX-II has been developed by Ford Motor Company and General Electric Company as a major element in the US Department of Energy's Electric and Hybrid Vehicle program. The ETX-II was quoted to be "the most advanced electric vehicle in operation today". Running in urban driving conditions as described in the FUDS driving schedule, the efficiencies of the electronic power inverter during driving was 97%, the motor efficiency was 87% and the transmission and wheel losses where rolling resistance was not included was 83%. During braking the efficiencies were 70, 72 and 44 %, respectively. When taken together, the combined efficiencies are 70% for driving and 22% for braking.
It is the primary purpose of this invention to provide electric powertrains for vehicles with lower losses. The invention permits designs where most types of losses in prior art designs are reduced compared to the prior art by providing a motor with a higher efficiency in a size and torque range permitting insertion inside the tire, by providing an improved design principle for inverters and by providing a improved fluid conductor switch design. The complete inventive design or parts thereof can be used in application fields other than power trains.
DC motors using permanent magnets is a motor type most likely to achieve very high efficiencies, since the magnetic field from the permanent magnets is obtained without the losses occurring in the field windings of other DC motors or the losses incurred by magnetization currents in induction or reluctance motors. Mechanically commutated motors require extra maintenance and incur voltage and/or friction losses. Brushless DC motors seems to be the most promising candidate for the high efficient vehicle motor. The ETX-II uses such motors.
The market for brushless DC motors in the range of several watts and higher is totally dominated by designs having stators where the windings of the three phases are overlapping. This means that the area circumvented by a coil belonging to one phase do not only contain a flux carrying iron pole; it will also circumvent slots containing windings belonging to other phases. This gives a less efficient use of pole iron mass and requires longer copper windings. Motor designs where the winding coils only circumvents flux carrying iron give a more efficient use of copper and iron and are more likely to provide the very efficient high-torque motors required. In order to be able to use loss reducing inverter circuits with a competetive production cost, the number of phases in the driving electronics should be reduced. To be able to combine this with good utilization of the rotor flux induced voltages in the stator windings, the flux changes in many poles should be synchronized. Pole groups where the poles have the same pitch as the rotor poles seem to be well suited to meet this requirement.
Most (or all) prior art electric power trains use fixed winding configurations. Conventional series/parallel configurations have been suggested. These however give loop currents in a parallel configuration. These loop currents give losses which, in the case of high efficient low resistance motors, will easily surpass the gains obtained. Also the resistance of the configuration switches will create losses.